In applications using a high voltage power supply, it is often desired to rapidly discharge the high voltage to a safe potential within a short period of time. Circuits designed to accomplish this are known as crowbar circuits or energy diverters. They are designed to quickly reduce a high potential to a low potential in order to prevent damage to equipment connected to the power supply, and also for the safety of those who may come in contact with exposed high voltage components.
An example of an application where crowbar circuits are desirable is with a helmet-mounted avionics display for military pilots. Helmet-mounted displays can require up to 20,000 volts to drive a cathode ray tube (CRT) in the display. Prior to disconnection of the helmet, such as during an emergency ejection, it is desirable to rapidly discharge the energy from the cockpit power supply in order to minimize the risk that an are or spark might be created in a potentially explosive environment. Not only must the potential be discharged, but it also must be discharged as rapidly as possible.
Different approaches have been used to construct crowbar circuits. As shown in FIG. 1, one prior art type of crowbar circuit uses a spark gap 28 to quickly discharge a high voltage source potential. A high voltage source 20 is connected to a cathode ray tube (CRT) display 26 via electrical connectors 22 and 24. Connectors 22 and 24 allow the display to be disconnected from the power supply. As noted above, before disconnecting display 26 from high voltage source 20 it is desirable to discharge the high voltage source potential to a lower potential, preferably near ground, in order to eliminate the risk that sparks or an arc will be generated when electrical connectors 22 and 24 are uncoupled.
When high voltage source 20 is to be discharged, i.e., when it is desired to uncouple the CRT, a trigger circuit 30 is used to apply a trigger pulse to the spark gap which can be a small canister filled with a normally nonconducting radioactive gas. The trigger pulse produces a small are within the spark gap, which begins to ionize the gas within the device. As the gas is ionized, the spark gap 28 becomes conductive, and current begins to flow from the high voltage source through the spark gap to ground. An avalanche effect is started, and the ionization increases with the increased current flow. This causes the high voltage potential to decay rapidly, as the initial charge is quickly channeled to ground. Once the current falls to a value insufficient to maintain arc conditions, current flow ceases and the gap returns to the nonconductive state.
While the spark gap crowbar circuit shown in FIG. 1 quickly discharges a high voltage source, it has several shortcomings. Physically, spark gap 28 is a fairly complex component containing a trace mount of radioactive tritium gas, for example. Because the rapid ionization of the gas quickly wears out the device, the spark gap must be periodically replaced at substantial cost. The spark gap also must be correctly sized for each application. Since a spark gap only operates within a narrow range of voltages, different spark gaps must be used in different environments. The operation of a spark gap is further limited in that a spark gap will not discharge a voltage that is less than 40% of its rating. As a result, designing with spark gaps is not an easy process.
Another disadvantage to using the spark gap circuit shown in FIG. 1 is that the quick discharge performed by the circuit produces voltage tinging. FIG. 3 is a graph depicting a typical spark gap's voltage ringing. The left vertical axis represents the voltage, and the lower horizontal axis represents time. A plot of the output potential across the spark gap is represented by a line 60. At time T.sub.1, the spark gap is triggered to discharge the high potential voltage. As shown by the near vertical slope of line 60 between times T.sub.1 and T.sub.2, the voltage quickly drops to near a ground potential. A typical spark gap crowbar circuit will be able to discharge a 13,000 volt potential in less than 15 nanoseconds. A result of this rapid discharge, however, is a voltage ringing induced between times T.sub.2 and T.sub.3. That is, the voltage across the spark gap partially rebounds before decaying down to a steady ground potential. In a typical spark gap device, the voltage rebound can be significant. The initial value of the ringing voltage may reach a level of 400 volts, a value that can cause component damage due to electrostatic discharge. In addition, the existence of a potential of even a few hundred volts can potentially harm user if sparks or other arcs are created by the crowbar circuit.
Another crowbar circuit design is a transformer-driven transistor array of the type shown diagrammatically in FIG. 2, which operates in the same environment as the circuits of FIG. 1. That is, high voltage source 20 is connected to display 26 via connectors 22 and 24. Instead of using a spark gap device, however, the crowbar circuit of FIG. 2 relies upon a number of transistor stages 40. A high voltage is applied across a number of these transistor stages, which are connected in a cascade configuration. In normal operation, the transistors are biased off so that no current will flow through the transistor stages 40. Each transistor stage 40 is coupled via a shared transformer 42 to a drive circuit 44. When a high voltage is to be discharged, the drive circuit is turned on, biasing each of the transistor stages 40 into saturation. When in saturation, each transistor stage 40 conducts current between the high voltage source and ground. Because all transistor stages are turned on simultaneously, the result is a rapid decay of a high voltage potential down to a ground potential. Typical transformer-driven transistor array circuits can discharge a 13,000 volt potential in under five microseconds. Once the drive circuit is turned off, the transistor bias potential is allowed to leak away, returning the transistors to nonconductive operation.
A disadvantage of a crowbar circuit constructed with a transformer-driven transistor array is that the transformer needed to simultaneously turn on all of the transistor stages 40 is a large and expensive device (the transformer must withstand the total output voltage). Additionally, because the circuit can discharge a high potential within less than five microseconds, the circuit also exhibits a voltage tinging as shown in FIG. 3. That is, much like a spark gap crowbar circuit, the rapid discharge of the high voltage source results in a voltage rebound of several hundred volts which could potentially impose a danger both to the circuit and to people nearby.